1. Field of the Invention
This invention relates to resin-bonded magnets which are one of the main members of permanent magnet motors widely used for controlling or driving, for example, peripheral equipment of computers, printers and the like. More particularly, the invention relates to a method for making such resin-bonded magnets which are comprised of ferromagnetic Fe-B-R alloys, in which R is Nd and/or Pr, and a resin composition. The resin-bonded magnet of the type to which the present invention is directed is described, for example, in U.S. Pat. No. 4,689,163.
2. Description of the Prior Art
As is known from and described in the above United States Patent, sintered ring or cylinder magnets of rare earth metal and cobalt alloys including, for example, Sm(Co, Cu, Fe, M)n, in which M is one or more elements of groups IV, V, VI and VII of the periodic table, and n is an integer of from 5 to 9, are very difficult in rendering them magnetically anisotropic along the radial direction of the ring. The main reason for this is considered due to the fact that the ring suffers a difference in expansion coefficient, based on the anisotropy, during the sintering process. Although the difference in the expansion coefficient is, more or less, influenced by the degree of magnetic anisotropy and the shape of the ring or cylinder, this generally has to be overcome by rendering the ring isotropic. This involves a disadvantage in that while the magnetic performance intrinsically reaches 20 to 30 MGOe in terms of maximum energy product, it lowers to about 5 MGOe along the radial direction of the ring or cylinder. Generally, the sintered magnet is mechanically brittle, so that part of the magnet is liable to come off and fly, with the fear that when such a magnet is applied to a permanent motor, a serious problem would occur with respect to the maintenance of their performance and reliability.
Resin-bonded ring magnets using rare earth metal and cobalt alloys can be made radially and magnetically anisotropic. This is because the difference in expansion coefficient between rare earth metals and cobalt is absorbed with the resin matrix. It is known that the resin-bonded magnet obtained by an injection molding has a maximum energy product of about 8 to 10 MGOe when rendered magnetically anisotropic along the axial direction. The resin-bonded magnet has a number of advantages: it has a density lower by approximately 30% than sintered magnets; the magnet can be designed to have a high dimensional accuracy; and because of the use of a resin, flexibility is imparted. Thus, it has generally been accepted that a resin-bonded ring magnet of Sm(Co, Cu, Fe, M)n undergoing radial magnetic anisotropy has well-balanced economy and performance as compared with sintered counterparts.
For the impartment of radial magnetic anisotropy to a resin-bonded magnet of a ring or cylindrical form, it is usual to generate a radial magnetic field in a cylindrical cavity accomodating the magnet. The radial magnetic field generator may be a generator which includes magnetic yokes and non-magnetic yokes arranged alternately to surround a mold, and a magnetizing coil provided outside the yokes as described, for example, in Japanese Laid-open Patent Application No. 57-170501, or a mold having a magnetizing coil embedded along the cavity. In order to cause a predetermined intensity of magnetic field to generate in the cavity, a high voltage, low current power supply is ordinarily used with a magnetomotive force being great. However, a magnetic path has to be so long as to cause a magnetic flux, produced by energization of the yokes with the magnetizing coil from the outer surface of the mold, to be effectively focussed within the cavity. Especially, with a small-sized cavity, a substantial amount of the magnetomotive force is lost or consumed as a leakage flux. Accordingly, it becomes difficult to make a resin-bonded magnet having a sufficiently radially magnetic anisotropy.
When used as a ring or cylinder magnet of radially magnetic anisotropy, a rare earth metal and cobalt alloy resin-bonded magnet may develop better magnetic characteristics than sintered ring or cylinder magnets. However, the magnetic characteristics of the resin-bonded magnet is greatly influenced by the shape of the magnet. This is a substantial and serious disadvantage when there is a strong demand for a small-sized and light weight resin-bonded magnet.